packet_filter.c

#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/skbuff.h>
#include <linux/netfilter_ipv4.h>
#include <linux/udp.h>
#include <linux/ip.h>
#include <linux/netdevice.h>

static char *interface = "eth0";
/* net filter hook option struct */
static struct nf_hook_ops netfilter_ops;
struct sk_buff *sock_buff;
/* udp header struct (not used) */
struct udphdr *udp_header;
/* ip header struct */
struct iphdr *ip_header;

unsigned int hook_func(unsigned int hooknum,
                       struct sk_buff **skb,
                       const struct net_device *in,
                       const struct net_device *out,
                       int (*okfn)(struct sk_buff *))
{
  if(strcmp(in->name, interface) == 0){

    sock_buff = *skb;

    /* grab network header using accessor */
    ip_header = (struct iphdr *)skb_network_header(sock_buff);



    if(!sock_buff) { return NF_ACCEPT;} 
    
    // Print the IP address
    printk("src IP addr:=%d.%d.%d.%d:%d\n", NIPQUAD(ip_headr->saddr)); 
    
    return NF_ACCEPT;
  }

  return NF_ACCEPT;
}

static int main(void)
{
  netfilter_ops.hook = hook_func;
  netfilter_ops.hooknum = NF_INET_PRE_ROUTING;
  netfilter_ops.pf = PF_INET;
  netfilter_ops.priority = NF_IP_PRI_FIRST;

  nf_register_hook(&netfilter_ops);

  return 0;
}

static void cleanup(void)
{
  nf_unregister_hook(&netfilter_ops);
}

module_init(main);
module_exit(cleanup)